The goal of our program is to explain the function of immunoglobulins in terms of their detailed three-dimensional structure. We are determining the crystal structure of several immunoglobulin light-chain dimers. We are refining the structures of proteins Cle, Sho, and Len using the simulated annealing program X-PLOR. X-PLOR is also used for determining additional immunoglobulin light-chain structures by molecular replacement. Refinement of the structure of the photosynthetic reaction center (RC) is also in progress using the computer program, X-PLOR. RC is among the very few membrane proteins for which the structure can be determined.